Due to increasing environmental (pollution) problems and accelerated energy depletion, there has recently been growing interest in a solar cell with high energy efficiency, which is obtainable from abundant natural resources and has no environmental pollution problems, as an alternative energy resource.
Such a solar cell generally has an absorption layer comprising a Cu—In(Ga)—Se film (often referred to as “CI(G)S based absorption layer”) as a coating layer containing a 1B-3A-6A group based compound so as to exhibit a high energy conversion rate and no deterioration caused by light irradiation, thus being considerably advantageous in the art.
The CI(G)S based absorption layer is typically fabricated by two kinds of conventional method, in particular, vacuum deposition and a method for applying a precursor under non-vacuum conditions followed by thermal treatment at high temperatures. The vacuum deposition may produce a high efficiency adsorption layer, however, may have problems such as decreased uniformity in manufacture of absorption layers having considerably large areas, requiring costly equipment, etc. On the contrary, the method for applying a precursor and thermally treating the same may provide an absorption layer with a large-scale uniform area, however, may entail reduced efficiency of the absorption layer.
Among methods for fabrication of an absorption layer using a precursor, for example, a method for applying a mixture paste of metal oxides to a surface of a substrate and post-processing the coated substrate to form an absorption layer has been disclosed. Although this method has a merit of preparing a uniform absorption layer at a low cost, the metal oxides used as a precursor are chemically and thermally stable so that large crystals may not be present in the final absorption layer, thus decreasing efficiency of a solar cell.
In order to solve the above problems in regard to conventional techniques, Korean Patent Laid-Open Publication No. 2005-82723 issued to the present applicant proposed a process for fabrication of a CIGS based absorption layer wherein copper selenide compound particles are mixed with indium selenide compound particles to prepare a mixture paste, the mixture paste is applied to a substrate, and the obtained coating layer on the substrate is rapidly heated to form the CIG based absorption layer. This process may increase a crystal size of the compound particle so as to allow fabrication of a high efficiency solar cell and may omit hydrogen reduction and selenide generation processes, thus simplifying the process of fabricating a CIGS based absorption layer for a solar cell.
In this regard, the present invention suggests a novel method for preparation of a compound containing group 6A element such as indium selenide, copper selenide, a composite thereof, and the like.
A conventional method for preparation of a CuSe2 compound may be exemplified by a mechanical alloying process which includes mixing of Cu powder and Se powder in a desired relative molar ratio and treating the mixture with a high energy ball mill (see, T. Ohtani, M. Motoki, K. Koh, K. Ohshima, Materials Research Bulletin, 30, (1995), 1495). However, this process has problems including, for example, neither control of specific reaction conditions nor detailed control of particle size and/or distribution of a product.
Another method for preparation of a CuSe2 compound has been disclosed, including: heating a CuCl2 solution in tri-n-octylphosphine (TOP) to 100° C.; introducing tri-n-octylphosphine oxide (TOPO) to the solution; adding tri-n-octylphosphine selenide (TOPSe) to the reaction mixture at 250° C. and reacting the same; and precipitating the reaction product in methanol to separate a copper selenide moiety, thus producing a final CuSe2 compound (see, H. Winkler, A. Brikner, V. Hagen, I. Wolf, R. Schmchel, H. V. Seggern, and R. A. Fischer, Advanced materials, 11 (17) 1444, 1999). However, the above method has a problem of using high price TOPO and TOPSe as a reaction raw material and a solvent, respectively.
Among conventional methods for preparation of an indium selenide compound, a chemical bath deposition (CBD) and a chemical method using tri-methylindium (TMI) as a reactant in a TOP solvent have been disclosed. The former has limitations such as difficulty in particle size and low reaction rate, while the latter entails problems of using high price TOP and unstable TMI as a reaction raw material and a solvent, respectively.
Meanwhile, some technologies using indium sulfate and/or selenious acid as a part of raw materials in the process of fabricating a CIGS based absorption layer have been disclosed. For instance, Japanese Patent Laid-Open Application No. 1997-036408 described a process for fabrication of an InSe compound layer used for a CIG based absorption layer involving electroplating in the presence of indium ions, selenious acid ions, sulfuric acid and sodium citrate. However, an electroplating based process exhibits reduced uniformity of the InSe compound layer formed on a substrate and requires high production costs compared to the previous method proposed by the present applicant, thus deteriorating physical properties of a final absorption layer. Therefore, the above process has not been applied in current mass-production thereof.
Accordingly, in consideration of various circumstances described above, there is still a strong demand for a process for preparation of group 6A element containing compounds such as metal selenides in fabrication of CI(G)S based thin films.